Semiconductor die attach operations are common in the industry and typically occur after the dicing or sawing of a semiconductor wafer into individual die. One common die attach operation involves the attachment of a semiconductor die to a packaging substrate, typically by way of a solder joint at a metal die pad, to produce a packaged die.
FIGS. 1 and 2 are illustrations of a typical prior art packaged die 100. As seen therein, a first surface of the die 101 (SEE FIG. 1) is attached in its entirety to a second surface of a die pad 102 with a die attach material 103 (see FIG. 1) such as solder. Bond pads 107 (see FIG. 1) are formed on a second surface of the die opposite the first surface. A set of bond wires 105 are attached to a bond pad on the die and to one of a set of leads 104 formed proximate to the die. The die, die pad, die attach material, bond wires and the inner portion of the leads are encapsulated by an encapsulant 106. As shown in FIG. 2, the encapsulant serves to hold the die, die pad, leads, and other components of the packaged die together in an integral structure that can be handled and processed as a unitary mass.
A packaged die of the type shown in FIGS. 1 and 2 is typically subjected to heating at one or more points during its manufacture. For example, during solder-based die attach, the die and the die pad are typically subjected to temperatures that are sufficiently high (e.g., greater than about 220° C.) to effect solder reflow. Such heating results in the presence of significant thermally induced stresses in the die. Similarly high temperatures are often encountered in adhesive die attach operations as well.
One type of thermally induced stress present in the die may be referred to as “differential CTE stress”, and arises from the mating together during die attach of materials having different coefficients of thermal expansion (CTEs). This effect is shown in FIGS. 3 and 4 for a die attach operation involving the attachment of a die 201 to a packaging substrate 203.
As shown in FIG. 3, at the assembly temperature, the die attach material 205, which is typically a solder or an adhesive, exists as a liquid. Consequently, the die 201 and substrate 203 are not rigidly bound together at this temperature, and may expand or shrink independently of each other as a function of temperature without the creation of stress. Below the solidification temperature of the die attach material, however, the die and substrate are more rigidly coupled together, and can no longer expand or contract independently of each other. Consequently, the presence of CTE differentials between the die and the substrate result in the presence of differential CTE stresses in the packaged die as the package is cooled below the solidification temperature, so that the cooling of the packaged die to room temperature after die attach results in the presence of significant differential CTE stresses in the packaged die. When the CTE of the packaging substrate is higher than that of the die, this cooling can cause bending in the packaged die as seen in FIG. 4.
The presence of differential CTE stress in the packaged die also induces shear stress and bending deformation. The amount of shear stress present in the package increases as a function of the cross-sectional surface area of the surfaces joined together by the die attach material. As shear stress increases, it may result in cracking or chipping of the die. The die is especially prone to cracking or chipping at places along its perimeter where it was sawn from the wafer, due to the presence of crack propagation sites there. An example of such cracking is shown in REGION 5 of the die in FIG. 5. This phenomenon can be a major source of yield loss in packaged die, especially in thinner die (e.g., die having a thickness of less than about 40 microns).
In order to achieve acceptable yields of the packaged product, the surface area of the die must typically be maintained below values that are optimal for many applications so that the attendant shear stress (and attendant yield loss) will not be excessive. Consequently, in such applications it is often necessary to utilize several smaller die, rather than one or a few larger die, thus complicating the assembly process. It is also often necessary in such applications to alleviate shear stress by choosing materials of more compatible CTE, but this frequently comes at a price of lower thermal performance due to the typically low thermal conductivity of such materials. Moreover, such an approach limits the use of metals such as copper for the die pad. However, the use of copper and other such metals in the die pad is otherwise advantageous due to the higher thermal conductivities and lower cost of these metals.
There is thus a need in the art for a method for relieving differential CTE stresses in die, especially during die attach. There is further a need in the art for a method for making a packaged die that permits the use of larger die and of die pad materials, such as copper, which have high thermal conductivities, without increased incidence of cracking or chipping in the die. These and other needs are met by the devices and methodologies described herein.